The use of polyester films for electrical insulation applications is known in the art. There are ever-more-stringent requirements for circuits to achieve complexity in a small space, and these circuits have to be capable of rapid, flexible, and low-cost production, the result being a high level of technological requirements placed upon the film and on the steps of further processing.
Conventional processes for the production of flexible circuits mostly relate to full-surface metallization of the film web, or lamination of the film web to metal foil, and then structuring of the resultant conductive surfaces by various methods, e.g. etching or mechanical removal of the undesired amounts of metal. There are also processes which apply the metal by stamping or printing, or a conductive paste comprised of carbon/silver is applied. Disadvantages of these processes are their low flexibility and precision, the result being a relatively large separation between the conductor tracks and/or the high costs associated with these processes, and/or the low production speeds.
EP-A-1 274 288 describes a process which, starting from copper-containing metal oxides, copper nuclei are formed via laser irradiation, and further copper is deposited onto these in a subsequent electroplating step. That specification describes exclusively injection moldings, and no indication is given as to whether and how the process can be applied for polyester films and films generally. In particular, there is no indication as to what types of polymers and additives are required to produce oriented films. The PBT/SiO2/spinel compounded material described in EP-A-1 274 288 is unsuitable for the production of oriented films, because the polybutylene terephthalate used has a high crystallization rate and a high fumed silica filler level, and is therefore not capable of reliable processing.
In particular, there is no indication as to how it is possible to produce a film with adequate dimensional stability under conditions of local heating after irradiation with electromagnetic radiation and further processing.
Another problem with films, due to low layer thickness, is that the absorption of radiation is insufficient, and therefore the amount of energy absorbed can sometimes be insufficient, to initiate the conversion of the metal salts into the elemental metals. A very high concentration of the activatable metal compounds therefore has to be added, and this is undesirable in the case of heavy metal compounds such as copper compounds, for environmental reasons.
Another particular problem with films is the high flexibility of the film web. The result is that the conductor track is subject to much higher mechanical stress than is the case with an injection molding. Adhesion to the metal is therefore very important. The prior art gives no indication relating to this matter.